1. Field of the Invention
The present invention relates to a process for the recovery of metal from ores which include substantial amounts of metal sulfides. More particularly, the present invention relates to the treatment of copper or lead sulfide ores for the recovery of the copper or lead as a substantially pure elemental metal in a single stage.
2. Prior Art
In the recovery of certain metals such as copper and lead from their respective sulfide ores, the metal frequently is found in the form of a sulfide. In addition to the specific metal of interest, such ores generally contain varying amounts of other metal sulfides. The generally available ores in the United States contain relatively low concentrations of such metals as copper and lead. Therefore, in the processing of such ores, it is the usual practice to form a concentrate or matte enriched in the metal of interest. Such concentrates may be formed by beneficiation or leaching of the ore to produce a concentrate richer in the desired metal constituents. In addition, or as an alternative thereto, the ore may be roasted or sintered to further concentrate the desired metal constituent.
The recovery of metals such as copper and lead from their naturally occurring sulfide ores is a complex process involving numerous process steps or stages. Sulfide ores are by far the most important ores of copper. They are usually complex mixtures of copper and iron sulfides associated with compounds of iron, zinc, arsenic, antimony, bismuth, selenium, tellurium, silver, gold and platinum. The principal copper sulfide or minerals are boronite, chalcopyrite, enargite, covellite and chalcocite. In the recovery of copper, the copper sulfide ore typically containing from about 1 to 5 wt % copper is first subjected to a concentrating step by either leaching the ore, using mechanical separation or employing flotation techniques to provide a copper concentrate containing typically from 15 to 40 wt % copper. The copper concentrate then is introduced into a roasting furnace to drive off the volatiles such as moisture and uncombined sulfur. The ore from the roaster now comprising about 25 to 45 wt % copper is introduced into a reverberatory furnace, wherein it is further treated to provide what is known in the trade as a copper matte, and which contains from about 40 to 45 wt % copper. The copper matte is introduced into a converter and reacted at elevated temperatures with air, wherein a part of the copper sulfide forms copper oxide which reacts with some of the remaining copper sulfide to yield elemental copper and a sulfur dioxide off-gas. The product from the converter generally is referred to as blister copper and contains from 98 to 99 wt % copper. The blister copper then is subjected to fire refining in the presence of a reducing agent to produce a product copper having a purity greater than 99%.
The processing of lead sulfide ores also involves a plurality of steps or stages. Galena (PbS) constitutes the most common and important of all the lead minerals. Where required, the lead sulfide ore is first subjected to concentrating techniques to provide an ore concentrate which typically will contain from 45 to 80 wt % lead. The lead concentrate then is subjected to a sintering operation to evolve some sulfur and other volatiles such as water. The product from the sintering operation is introduced into a blast furnace where it is reacted with coke to reduce the lead and produce an off-gas containing SO.sub.2. The lead product from the blast furnace, referred to as bullion, will contain from about 94 to 98 wt % lead. The bullion is introduced into a drossing furnace, where it is fluxed with an alkali metal salt such as sodium carbonate to remove residual impurities. The product from the drossing furnace, usually referred to as a dross, comprises 99 wt % lead. The dross usually is subjected to a softening operation wherein other constituents of the ore, such as tin, antimony, and arsenic, are volatilized and driven off and a final lead product having a purity of about 99.9% is obtained.
Numerous descriptions of the various steps of metal-refining processes are found in standard references as well as in the patent literature.
An example of lead-smelting process is found in U.S. Pat. No. 821,330. The patent discloses a process for reducing the lead sulfide ore which comprises smelting it, preferably in a reverberatory furnace, with carbon, iron oxide, and sodium carbonate to produce metallic lead and a fusible iron-sodium matte.
In U.S. Pat. No. 862,378 there is disclosed a method of smelting and refining copper ores. The copper ore, containing combustible impurities in an amount sufficient to generate the required heat necessary in the smelting operation without the addition of carbonaceous fuel, is first subjected to a combined smelting and converting operation. A copper matte is produced which is then transferred in molten form to a converter vessel where it is subjected to an air blast in the presence of siliceous ores to eliminate the iron contained in the matte. In the converter a slag containing various impurities is formed and removed, leaving a blister copper which is then transferred in molten form into a preheated refining furnace for elimination of any residual oxides and production of essentially pure copper.
U.S. Pat. No. 2,823,990 relates to a process for the treatment of lead ores. An ore concentrate, containing more than 40 wt % lead in the form of lead sulfide, in admixture with soda and carbon, is wetted with water and smelted in a flame-fired rotary furnace at a temperature between 700.degree. and 1000.degree. C. to recover a high purity lead.
U.S. Pat. No. 919,130 discloses a process for extracting copper from its ores. The process comprises mixing a copper ore with a carbonaceous material and carbonates of the alkali and alkaline earths and subjecting the mixture to heat in a closed retort to produce metallic copper.
More recently in U.S. Pat. No. 2,834,669, there is disclosed another process for the extraction of a metal such as lead from its sulfide. The process comprises melting lead sulfide in the presence of sodium carbonate, an oxide selected from the group consisting of zinc oxide, iron oxide, silica and lime, sodium chloride and carbon, thereby causing reduction of lead sulfide to lead.
U.S. Pat. No. 3,899,322 relates to a molten salt process for the recovery of certain noble-type metals (such as copper or silver) from scrap containing the same. The process comprises feeding the noble-type metal value-containing scrap and a source of uncombined gaseous oxygen into a pool of molten salt consisting essentially of sodium carbonate and containing about 1 to 25 wt % sodium sulfate. The temperature of the molten salt is maintained between 800.degree. and 1800.degree. C. and above the melting point of the noble-type metal to thermally decompose the scrap to form separable scrap decomposition products and said metal. This patent however does not describe the treatment of sulfide ores or mattes derived therefrom.
Other patents which relate to the combustion of carbonaceous fuels and carbon-containing waste in a molten alkali metal salt for various purposes are known. U.S. Pat. No. 3,710,737 shows the generation of heat for external use employing a variety of carbonaceous materials. U.S. Pat. Nos. 3,567,412; 3,708,270; and 3,916,617 show the use of such techniques for the production of pyrolysis gases. In U.S. Pat. Nos. 3,778,320 and 3,845,190 such techniques are involved, respectively, in the nonpolluting disposal of explosives and of organic pesticides. None of these patents are concerned with the recovery of metals from their sulfide ores.
In many of the prior art processes wherein sulfide ores are roasted or sintered, a significant amount of gaseous sulfur dioxide is produced and constitutes a potential pollutant. Further, in subsequent refining of the ore concentrates or mattes, many typical processes evolve additional sulfur dioxide. Public concern regarding the quality of the environment has resulted in legislation regulating the amount of pollutants, such as sulfur dioxide, which may be discharged into the atmosphere.
Clearly, there exists a need for an economical, nonpolluting, single stage process for the recovery of metals from their sulfide ores in a substantially pure elemental form.